Scattering HCl Molecules from Au(111) and Ag(111) Surfaces

In this thesis, I present an experimental molecular beam surface scattering study of dynamical processes involved when HCl molecules are scattered from Au(111) and Ag(111) surfaces. I investigated vibrational excitation, translational inelasticity and dissociative adsorption in combination with associative desorption. The experiments were conducted in an ultra-high vacuum (UHV) molecular beam/surface scattering apparatus equipped with a pulsed nanosecond infrared (IR) laser source for vibrational state manipulation and pulsed nanosecond ultraviolet (UV) lasers for quantum-state-resolved detection of molecules via resonance enhanced multi-photon ionization (REMPI) before and after the collisions. For HCl/Au(111), I found surface temperature dependent vibrational excitation probabilities (VEPs) from vibrational state v… = 0 ->†’ 1 to be in the range of 10^-ˆ’5 -ˆ’ 10^-3 for incidence energies of Ei€‰ = 0.67 -ˆ’ 0.99 eV, which is low compared to other molecule-surface systems. On the other hand, VEPs for the v = 1 ->†’ 2 transition were substantially higher at 10^-ˆ’3 - 10^-2 for comparable incidence energies. In both cases, excitation probabilities could be divided into electronically adiabatic and nonadiabatic contributions where the latter exponentially depended on the surface temperature Ts. This combination of adiabatic and nonadiabatic vibrational excitation has so far been uniquely observed for HCl scattered from Au(111) and Ag(111) surfaces. Extracting ˆEi and Ts independent interaction coefficients, I found the nonadiabatic excitation to be stronger than the adiabatic one by a factor of 67 for the v = 0 -> †’1 and 24 for the v = 1 -> †’2 channel. In comparison, on Ag(111) only the excitation from the vibrational ground state could be observed. With VEPs in the range of 10^-4 - 10^-3 being slightly higher than on gold, this difference was interpreted as enhanced nonadiabatic interactions on silver. Relatively low barriers to dissociation on both metal surfaces (e. g., compared to NO/Au(111)) might have led to the higher v = 1 -> 2 VEPs on Au(111). Further, the absence of higher excitation channels (v = 2 ->†’ 3 on gold, v = 1 -> 2 on silver) might also be explained by the enhanced dissociation of HCl molecules incident in excited vibrational states. Dissociation probabilities of HCl on metal surfaces were measured by AUGER electron spectroscopy after controlled molecular beam dosage monitored by a quadrupole mass spectrometer. Even though the dissociation probabilities on Ag(111) were predicted to be higher than on Au(111), I could not determine them within the current technical limitations. Additionally, the actual dissociation probabilities I determined on Au(111) employing AES remained below 0.06 -ˆ’ 0.17, which is lower than those predicted by a series of computational studies by a factor of 2 -ˆ’ 6. On the other hand, translational energy losses calculated in the latest AIMDEF study match the experimental results for v = 1 -> 1 and v = 1 ->†’ 2 quite accurately. In sum, the results of this thesis, which all indicate that HCl exhibits both electronically adiabatic and nonadiabatic interactions with metal surfaces during scatter- ing events, can serve as benchmark data to test and improve theoretical descriptions of gas-surface interactions, especially in those cases were nonadiabaticity plays an important role.

Wodtke, Alec
Beck, Rainer
Lausanne, EPFL

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 Record created 2019-10-14, last modified 2020-04-21

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